lec 6 other detectors-2
TRANSCRIPT
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Pakistan Institute of Engineering & Applied Sciences(PIEAS)
Dr. Nasir M Mirza
Deputy Chief Scientist,
Department of Physics & Applied mathematics,
PIEAS, P.O. Nilore, 45650, Islamabad.
Email: [email protected]
Ph: +92 51 9290273 (ext: 3059)
Lectures onRadiation Detection
Delivered in Professional Training Course onRadiation Safety & RWM at PNRA (April, 2008)
mailto:[email protected]:[email protected] -
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Other Types of Radiation Detectors
Recommended Text Books
1. Glenn F Knoll sRadiation Detection & Measurement (recentedition).
Lecture 6
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Personal Monitoring
Individual monitoring is needed external radiation measurements: The worker wears a dosimeter from the followings
Ion-chambers and Pocket dosimeter
Stray radiation chambers
TLD (thermo luminescent dosimeter)
Electronic dosimeter
Film Badges
Photographic films
MOSFETs
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TL Response
Variation with gamma-ray energy of the TL response per roentgen
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TLDconti.
Oven for temperature annealing
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TLD Readers
TLD reader
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Personal monitoring with TLD
One or more small pieces of TL
material are placed in a holderthat is worn by a person beingmonitored for a prescribed time
After that TLD is heatedelectrically
Intensity of luminescence ismeasured with PMT
PMT output is read by digital
voltmeter in units of charge(nC)
The instrument is calibratedwith known dose of radiation
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Environmental monitoring with TLD
Environmental radiation doses
are much lower than the work
place doses
Require much more sensitive
material than for radiation
workers
Al2O3:C can measure does as
low as 0.1 SV (0.01m rem)
High degree of sensitivity
The best choice forenvironmental dosimetry
TLD reader
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Various Types of TL
Thermoluminescence materials are available in powder forms, and in
accurately sized discs, rods, chips, and cubes
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ELECTRONIC DOSIMETERS
Features
Smaller, more reliable, and moresophisticated instrument, lightweight (~100 g), accuratedosimeter
More useful for personal
dosimetery than film or TLD Measure and display
instantaneous dose rate andintegrate over time to obtain,store, display accumulated dose
Design to alarm at any desireddose rate or accumulated dose
Data can be stored in memorycan be down load into computerfor dose tracking or record
keeping
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Electronic dosimeter
Direct reading electronic
dosimeter for personal monitoring
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Electronic Dosimeters conti.
Material
Solid- state semiconductors, silicone diodes;
Detect and radiation over a wide range of dose rates
and doses.
137Cs: dosimeter with 10% accuracy can
Measure dose rate from 1 Sv/hr to 10 mSv/hr
Can store dose from 1 Sv to 10 Sv.
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Electronic Dosimeters conti.
Working Electronic Pocket dosimeter
Incorporate several detectors
Each one respond to radiation that has passed through
different filters Output of several detectors is added which is
proportional to the absorbed dose
Response, energy independent ( 25%) from 60 keV
1.5 MeV
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Geiger-Mueller Counter
It is Not a dosimeter It is just a
counter of radiation events;
Very sensitive;
Light weight and convenient to use
Suitable for miniaturization
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Geiger-Mueller (G-M) Devices
Useful for
Area monitoring
Room monitoring
Personnel monitoring
Care required in regions of high dose rate or pulsed beamsas reading may be inaccurate
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SURVEY METERS:
Response is proportional to absorbed energy
Have air equivalent wall measure exposure
Calibrated to read in units of mR/h, C/kg/h, Sv/h
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Film Badges
The sensitive element
is a strip of 16-mm film(bottom) that is sealed into a
light-tight envelope and is
placed inside a holder(top).
Energy dependence of the
film is overcome through the
differences in film darkening
under the open window and
under several different filtersthat are built into the holder.
FIGURE : Film badge dosimeter for X-rays, gamma rays, and beta rays.
Holder
Film
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Film Badges
FIGURE :
Relationship between
radiation exposure and optical
density.
CurveA is the response ofduPont type 555 and
CurveB is the response of
duPont type 834 dosimeter
film to 60Co gamma rays.
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Radiographic film
Reduction of silver halide to silver
Requires processing ---> problems with reproducibility
Two dimensional dosimeter
High spatial resolution
High atomic number ---> variations of response with
radiation quality
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Radiographic film
Cross section
Often prepacked
for ease of use
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Film: dose response
Evaluation of film via optical
density
OD = log (I0 / I)
Densitometers are commercially
available
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Radiographic film dosimetry in practice
Depends on excellent processor
QA
Commonly used for demonstration
of dose distributions
Problems with accuracy and
variations in response with X Ray
energy
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Radiochromic film
New development
No developing
Not (very) light sensitive
Better tissue equivalence
Expensive
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Semiconductor Devices
Diodes
MOSFET detectors
Diodes for water phantom
measurements
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Diodes
From Metcalfe et al. 1996
Mostly used like
a photocell generating
a voltage proportional
to the dose received.
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Metal Oxide SemiconductorField Effect Transistor
From Metcalfe et al. 1996
MOSFETs = extremely
small sensitive volume
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1. irradiation
2. Charge
carriers trapped
in Si substrate
3. Currentbetween source
and drain altered
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Diodes and other Solid State Devices
Advantages
direct reading
sensitive
small size
waterproofing possible
Disadvantages
temperature sensitive
sensitivity may change --> re-
calibration necessary
regular QA procedures need
to be followed
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Summary of lectures
Ion chambers Semiconductors TLDs Film
Advantages Well understood,
accurate, variety of
forms available
Small, robust Small, no cables
required
Two dimensional,
ease of use
Disadvantages Large, high voltage
required
Temperature
dependence
Delayed readout,
complex handling
Not tissue
equivalent, notvery reproducible
Common use Reference
dosimetry, beam
scanning
Beam scanning, in
vivo dosimetry
Dose verification,
in vivo dosimetry
QA, assessment of
dose distributions
Comment Most common andimportant
dosimetric
technique
New developments(MOSFETs) may
increase utility
Also used fordosimetric
intercomparisons
(audits)
New developments(radiochromic
film) may increase
utility